Importing PNG files into Blender Grease Pencil
In short... You can't.
PNG images are only images. You can't animate them in any program without previously rigging or doing most of the work by yourself.
There is an addon called Import Images as Planes that allows to import PNG images to blender as a plane in a 3D space. I could see a little animation made this way.
But png to grease pencil is pretty much impossible without transferring this manually to curves or redrawing it yourself.

Hope this helped.